The present invention relates to a silicon carbide-based porous body. More particularly, the present invention relates to a silicon carbide-based porous body which can avoid excessive temperature elevation when it is used as a filter and the particulate matter (PM) captured thereby is burnt and removed.
There have been widely used a filter [a diesel particulate filter (a DPF)] for capture and removal of the particulate matter present in a dust-containing fluid (e.g. an exhaust gas emitted from a diesel engine), and a porous honeycomb structure as a catalyst carrier for loading of a catalyst component capable of purifying the harmful substance present in an exhaust gas. Also, it is known to use, as a constituent material for such a honeycomb structure, refractory particles such as silicon carbide (SiC) particles. There is disclosed, for example, a porous silicon carbide-based catalyst carrier of honeycomb structure, obtained by using, as a starting raw material, a silicon carbide powder having a particular specific surface area and a particular impurity content, forming it into an intended shape, drying the formed material and firing the dried formed material at a temperature range of 1,600 to 2,200° C. (see, for example, Patent Literature 1). There are also disclosed a porous honeycomb structure comprising refractory particles (an aggregate) (silicon carbide, in particular) and metal silicon (see, for example, Patent Literature 2) and a method for production thereof. There is further disclosed a DPF using cordierite as a constituent material for honeycomb structure (see, for example, Patent Literature 3).
Meanwhile, there is disclosed a filter for diesel engine exhaust gas, having a gas-incoming filter made of a ceramic of low thermal conductivity and a gas-outgoing filter made of a ceramic of high thermal conductivity, the two filters being connected lengthwise in a direction in which an exhaust gas enters into the gas-incoming filter (see, for example, Patent Literature 4).    Patent Literature 1: JP-A-1994-182228    Patent Literature 2: JP-A-2002-154882    Patent Literature 3: Japanese Patent No. 2578176    Patent Literature 4: JP-A-1995-204431
The particulate matter (PM) discharged from a diesel engine is captured by a DPF installed in the diesel engine. However, the captured PM need be burnt and removed when the amount of the PM has reached a certain level, because the capture of PM causes a rise in pressure loss. Since the exhaust gas discharged from a diesel engine is at a low temperature of not higher than 300° C., the DPF need be heated to burn the PM. Various methods have been investigated for DPF heating and, in recent years, there has been selected (employed) a method of burning unburnt hydrocarbons with an oxidation catalyst provided at the inlet portion of DPF to overheat the DPF inlet by the combustion heat, of the unburnt hydrocarbons. The silicon carbide-based porous body disclosed in the Patent Literature 1, produced by the sintering brought about by the recrystallization reaction of silicon carbide powder per se has had a problem that the porous body has a high thermal conductivity at low temperatures (about 200° C.), making large the amount of the unburnt hydrocarbons required before the PM reaches its combustion temperature and incurring inferior fuel consumption. The silicon carbide-based porous body disclosed in the Patent Literature 2, containing silicon carbide (an aggregate) and metal silicon, has had the same problem as above that the porous body has a high thermal conductivity at low temperatures (about 200° C.), making large the amount of the unburnt hydrocarbons required before the PM reaches its combustion temperature and incurring inferior fuel consumption; moreover, the porous body has a small heat capacity at high temperatures, causing excessive temperature elevation owing to the combustion heat of PM and posing a risk of loss by melting. Meanwhile, the cordierite DPF disclosed in the Patent Literature 3 has a low thermal conductivity at low temperatures and reaches the combustion temperature of PM with a smaller amount of unburnt hydrocarbons; however, it has a small heat capacity at high temperatures, causing excessive temperature elevation owing to the combustion heat of PM and posing a risk of loss by melting. Further, the filter for diesel engine exhaust gas, disclosed in the Patent Literature 4, having a gas-incoming filter made of a ceramic of low thermal conductivity and a gas-outgoing filter made of a ceramic of high thermal conductivity has a bonded structure and has had a risk of PM leakage at the bonded portion.
The present invention has been made in view of the above-mentioned problems, and aims at providing a silicon carbide-based porous body which can avoid excessive temperature elevation when it is used as a filter and the particulate matter (PM) captured thereby is burnt and removed.